The long-term objective of this proposal is to develop new photoaffinity labels for the mu opioid receptor which will materially assist in the identification, characterization and sequencing of the mu receptor. The photoaffinity labels are designed to photolyze at about 35 nm to minimize photodestruction of the target receptor. One group of mu ligands will contain either a cyclohexenone or cyclohexadienone functionality and have not been employed previously for photolabelling of opioid receptors. The second group of compounds will consist of opiates with intrinsic photoreactivity. Preliminary experiments demonstrated that a codeinone derivative photolysis in methanol at 365 nm to give a new compound which has incorporated the elements of methanol. All target compounds will be submitted for receptor binding and in vitro and in vivo opioid activity. Ligands which photolabel the mu receptor will be radiolabelled for the purpose of tagging the receptor.